1. Field of the Invention
This invention relates to improvements in the regeneration of fluid catalytic cracking catalyst. More particularly, it relates to reducing fluid catalytic cracking regenerator temperature, catalyst emission losses and carbon on regenerated catalyst. This invention especially relates to the use of oxygen enriched regeneration gas and the cooling of regenerated catalyst in a fluid catalytic cracking unit.
2. Description of the Prior Art
The fluidized catalytic cracking of hydrocarbons is well known in the prior art to produce gasoline, heating oil and diesel fuel. Normally in such processes a hydrocarbon charge, such as a vacuum gas oil, is contacted with hot, regenerated solid catalyst particles either in a fluidized bed reaction zone or an elongated riser reaction zone under cracking conditions for conversion of the hydrocarbon charge into cracked hydrocarbon products with the concommitant deposition of carbonaceous materials (coke) upon the catalyst; separating the cracked hydrocarbon vapors from the coke contaminated catalyst (spent catalyst) within the reaction zone, recovering the cracked hydrocarbon vapors as product essentially free from entrained catalyst, stripping volatile hydrocarbons from the spent catalyst by contact with stripping vapors in a stripping zone; regenerating the coke contaminated stripped catalyst in a regeneration zone by burning coke from the catalyst with a molecular oxygen-containing regeneration gas at an elevated temperature for restoring activity to the regenerated catalyst; and contacting hot, regenerated catalyst with additional hydrocarbon charge in the reaction zone, as described above.
The catalyst and the hydrocarbon charge are maintained at an elevated temperature for a period of time sufficient to effect the desired degree of cracking to lower molecular weight hydrocarbons typical of those present in motor gasoline and distillate fuels.
During the cracking reaction, coke is deposited on the catalyst particles in the reaction zone thereby reducing the activity of the catalyst for cracking and the selectivity of the catalyst for producing gasoline blending stock. In order to restore a major portion of its activity to the coke contaminated or spent catalyst, the catalyst is transferred from the reaction zone to a regeneration zone wherein the catalyst is contacted with a molecular oxygen-containing regeneration gas, such as air, under conditions sufficient to burn at least a portion, preferably a substantial portion, of the coke from the catalyst. The regenerated catalyst is subsequently withdrawn from the regeneration zone and is reintroduced into the reaction zone for reaction with additional hydrocarbon feed. Commonly, spent catalyst from the reaction zone is passed therefrom to a stripping zone for removal of strippable hydrocarbons from the catalyst particles prior to transferring the catalyst to the regeneration zone.
Typically, regeneration zones of the prior art comprise large vertical cylindrical vessels wherein the spent catalyst is maintained as a fluidized bed by the upward passage of a molecular oxygen-containing regeneration gas. The fluidized catalyst forms a dense phase catalyst bed in the lower portion of the vessel and a dilute catalyst phase containing entrained catalyst particles in the upper portion of the vessel with an interface existing between the two phases. Flue gas, which normally comprises gases arising from the combustion of the coke on the spent catalyst, inert gases such as nitrogen from air, any uncoverted oxygen and entrained catalyst particles, is then passed from the dilute catalyst phase into a solid-gas separation means within the regeneration zone to prevent excessive losses of the entrained catalyst particles. The catalyst particles separated from the flue gas are returned to the dense phase catalyst bed. A substantially catalyst-free flue gas may then be passed from the separation means to equipment downstream thereof, such as a plenum chamber, for discharge from the top of the regeneration zone. Cyclone separators are commonly used as such separation means.
The fluid catalytic cracking processes in use today conventionally employ catalysts of the crystalline aluminosilicate type. Such catalysts usually comprise an amorphous matrix such as silica-alumina and the like plus a minor portion of a crystalline aluminosilicate having uniform pore openings which have been ion exchanged with rare earth ions, magnesium ions, hydrogen ions, ammonium ions and/or other divalent and polyvalent ions for reduction of the sodium content of the crystalline aluminolsilicates. These cracking catalysts are conventionally known in the art as "zeolite catalysts" and are commercially available. Although these catalysts are subject to physical change, manifested as loss of crystallinity, from exposure to excessively high temperatures, it has been found that they may be subjected to temperatures of up to about 1500.degree. F. without substantial damage to the physical structure of the catalyst. Thus, regeneration process variables may be adjusted to properly achieve the desired residual carbon content on the regenerated catalyst within the above temperature limitation.
Since hydrocarbon cracking is an endothermic process and the burning of the coke from the spent catalyst is an exothermic process, most fluid catalytic cracking processes are designed to be heat balanced. That is, the burning of the coke in the regenerator supplies enough heat, even taking losses into account, to satisfy all of the heat requirements of the reactor. There is a definite relationship between the amount of coke produced during cracking, the amount of coke burned off during the regeneration and the heat which the reactivated heated catalyst returns to the cracking process effected in the reactor. Even this combination of relationships is not wholly independent and controllable because they are in turn partially influenced by the nature of the catalyst and its tendency to make more or less coke under given cracking conditions, as well as the nature of the petroleum fraction feed and its tendency to be converted to more or less coke under a given set of cracking conditions.
It has been, in the past, the usual commercial practice to carefully balance all of the effects and countereffects in a fluid catalytic cracking system and to adjust feeds, residence times, catalyst and other operating conditions to achieve a heat balanced operation. Thus, the type of feed, the feed rate, the feed temperature, the type of catalyst, catalyst to oil ratio, contact time, reaction temperature and other variables are adjusted on the cracking side so as to produce as desirable a product slate as possible while depositing a sufficient amount of coke on the catalyst to satisfy the heat requirements of the system. In the regenerator, adjustments are made to the air inlet temperature, rate of air flow, ratio of CO.sub.2 to CO and flue gas composition to provide the required heat balance.
Increasing or varying oxygen content of the regeneration gas introduced to the regenerator has been employed heretofore. U.S. Pat. No. 2,225,402 of Liedholm discloses improved catalyst reactivation utilizing a space velocity of at least 30 through the catalyst, a minimum pressure of at least two atmospheres and an oxygen concentration varying from 2.1 mole % at the beginning of the regeneration of 1.2% at the end. U.S. Pat. No. 3,563,911 of Pfeiffer et al. discloses a multistage fluidized catalyst regeneration process employing air, oxygen or oxygen-enriched air in all of the several stages. U.S. Pat. No. 4,176,084 of Luckenbach teaches that the poisoning effect of metal contaminants deposited on a catalyst may be reduced by periodically increasing the oxygen in excess of that which is required to completely burn the coke to CO.sub.2. This may be achieved by increasing the rate of oxygen charged to the regenerator while maintaining the fresh feed and catalyst circulation rates constant or by maintaining the oxygen rate through the regenerator constant while decreasing the fresh feed and catalyst recirculation rates. This excess oxygen introduction may be employed for periods of up to 20% of the regeneration time.
Catalytic cracking catalyst coolers have been employed heretofore. U.S. Pat. No. 2,386,491 of McOmie discloses the use of a catalyst cooler located external to the regenerator while U.S. Pat. No. 3,223,650 of VanPool uses indirect heat exchange coils within the regenerator. U.S. Pat. No. 4,064,039 of Penick removes the excess heat generated during the regeneration of a coked catalyst which has incorporated therein a platinum group metal to promote the conversion of CO to CO.sub.2. In Penick the hot regenerated catalyst may be cooled by either direct or indirect means and may be effected either within the regenerator or external thereto. In one preferred technique a portion of the catalyst is withdrawn from the regenerator, passed through an indirect water cooled heat exchanger and reintroduced into the regenerator. In another configuration, the hot regenerated catalyst is subject to indirect heat exchange while it is passing from the regenerator back to the reactor. In still another embodiment internal cooling of the catalyst in the regenerator is accomplished by providing water cooling for the cyclone separators in the upper portion of the regenerator vessel.
The operation of the regenerator in many fluid catalytic cracking units is presently constrained by metallurgical temperature limitations, air blower capacity and/or excessive catalyst losses due to high vapor velocities within the regenerator. As feedstocks to fluid catalytic cracking units become heavier, regenerator capacities to handle the increased coke loading will further limit the throughput or the severity of these fluid catalytic cracking units.
It is an object of this invention to reduce fluid catalytic cracking regenerator operating temperatures, reduce catalyst emission losses and reduce the carbon on regenerated catalyst.
It is another object of this invention to operate a fluid catalytic cracking unit at a higher conversion rate, at a higher fresh feed rate and/or with feed streams of higher coke-making tendencies.